Background
In light of the planet's significant warming over the last few decades of the 20th century, it is only natural to presume that many plants concurrently expanded their ranges in a northwards direction, providing for more overlapping of individual ranges and concomitant increases in local species diversity. One way of documenting this northward expansion of vegetation is what the authors call "the long tradition of tree-line research." Now, however, they augment that older approach with an analogous study of the range expansion of shrubs.

What was done
Working at a site just three kilometers from the Abisko Scientific Research Station (68°21'N, 18°49'E) in the Northern Swedish Scandes, Hallinger et al. studied male plants of the medium-sized Juniperus nana shrub, collecting the main stems of five to eight shrubs every hundred meters of elevation until the shrub zone ended. Ring-width measurements on these stems were then performed, as they describe it, "to measure radial and vertical growth, to track growth changes over time, to age the shrub individuals and to correlate annual shrub growth with climate," the characteristics of which latter factor were derived from records of the nearby Abisko Station.

What was learned
The three researchers report that they "documented a distinct increase in radial and vertical growth rates of J. nana shrubs during recent decades in the subalpine zone of North Sweden," and they say that "the age structure of shrubs along the elevational gradient provides evidence that an upslope advance of the altitudinal shrubline is underway." In addition, they state that they "observed significant, strong and stable correlations between annual ring width and summer temperatures (June, July, August)," and that "the acceleration of radial and vertical growth since 1970 also coincides with the recent three decades of rising arctic air temperatures and the warming trend of 0.2°C per decade for the average temperature since 1956 at Abisko."

What it means
The fruits of the German scientists' work add to what they call the "mounting evidence that shrubs are expanding into alpine and arctic areas because of climate warming," and they note that "this expansion occurs in both evergreen and deciduous shrub types," citing the additional findings of Forbes et al. (2009). And as a result, as we like to remind everyone -- whenever possible -- the great warming- and CO2-induced greening of the earth continues.

Reference
Forbes, B., Fauria, M. and Zetterberg, P. 2009. Russian Arctic warming and greening are closely tracked by tundra shrub willows. Global Change Biology 15: 1-13.